This proposal focuses upon the potential role in stress pathology of a novel neurosecretory mechanism involving the co-expression and release of synergistic neuroactive peptides from hypothalamic cells. Experiments will combine a behavioral analysis of chronic long-term stress states with quantitative immunocytochemical (ICC) analysis of neurosecretory cells. We shall concentrate on neurons whose normal responsibility is the release of corticotropin releasing factor (CRF) and hence the regulation of the pituitary- adrenal axis. Certain CRF neurons are able to co-express a synergist, namely vasopressin (VP) or oxytocin (OT). When a physiologically normal rat is stressed for 72 hrs by exposure to alternating periods of white noise and conditioned emotional stimuli, the number of cells in the hypothalamic paraventricular nucleus co-expressing CRF with OT increases. After 240 hours of stress exposure, the number, size and staining intensity of CRF cells increase. To determine if the stress-induced changes occur in neurosecretory cells afferent to the median eminence (ME), retrograde tracing from the ME will be combined with ICC. The degree of stress-induced activation of CRF neurons and anterior pituitary cells will be assessed by cytochemical detection of the enzyme cytochrome oxidase as a probe of cellular metabolic history. Our primary interest is whether systematic variations in the parameters of stress will cause lasting alterations in the degree and/or pattern of peptidergic expression in hypothalamic neurons. Behavioral paradigms will be employed to analyze the relative contributions of such factors as anticipatory anxiety, vigilance, predictability, and the availability of coping responses. A combined behavioral and immunocytochemical analysis should allow us to determine whether the plasticity of hypothalamic neurosecretory cells represents an endocrine potentiating mechanism that plays a role in the normal regulation of behavior under stress and/or in the etiology of stress-related disorders.